Process for Electroless Deposition on Magnesium Using a Nickel Hydrate Plating Bath

ABSTRACT

A plating process using an electroless plating bath formed from a single component solution used to effect nickel or nickel alloy plating on magnesium. The plating solution is provided with a nickel hydrate compound as a source of plating ions, a sodium hydrate compound and ammonium hydroxide. The bath solution is heated to a temperature of 50° C. or more, and the substrate is immersed therein for upto ten minutes to allow for formation of the heating layer.

RELATED APPLICATIONS

This application claims the benefit under 35 USC §119(e) to U.S. Provisional Application Ser. No. 61/599640, filed Feb. 16, 2012.

SCOPE OF THE INVENTION

The present invention provides a method and process of electroless coating of magnesium and magnesium alloy substrates with metal coatings and more preferably, coatings of nickel phosphorus (Ni—P) and/or its alloys, including zinc (Zn) in the form of (Ni—Zn—P) and (Ni-p-Zn) and/or Tungsten (W) in the form of (Ni—W—P), as for example part of a method of preventing the galvanic corrosion of magnesium.

BACKGROUND OF THE INVENTION

Magnesium is the eighth most abundant metal on earth. Magnesium is two-thirds the weight of aluminum; whilst having physical and mechanical properties approaching that of steel; and is easy to form and machine. Heretofore, the use of magnesium in commercial metal fabrication applications has been limited due to its extreme susceptibility to galvanic corrosion, particularly when it is placed in contact with metals other than aluminum and zinc, in the presence of electrolytes.

The electroless coating of metals is a widely used process that has been in existence for some time. Conventional electroless coating studies have generally found that the deposition of coating metals in electroless coating processes falls off dramatically as the pH of the coating solution approaches 10 or higher. In particular, it has therefore been generally accepted that electroless coating is so excessively slow, that it cannot be commercially achieved at pH levels of 11 or higher. In addition, in electroless coating processes, to obtain a good intermetallic bond between the coating material and the parent substrate, it is necessary that the substrate be clean; free of all foreign material; and free of any oxidation. Oxidized metal surfaces are generally understood as unsuitable bonding sites for any metal, including metals deposited by electroless coating processes. These factors are even more pronounced when attempting to coat magnesium using electroless processes.

When magnesium is coated using conventional electroless coating baths, the bath chemistry has been found to promote the oxidation and/or corrosion of the surface of the magnesium prior to any coating metal deposition. This in turn may result in the formation of a poor finish, poor adhesion around oxidized and corroded areas, and/or intermittent coverage of the coating layer over the magnesium surface. Further, with conventional plating baths where magnesium oxide remains present on the substrate surface, little or no deposition frequently occurs in oxidized locations.

One earlier process developed by the inventors provided for the deposition of a copper intermediate layer onto magnesium substrate using a high pH (i.e. a pH of about 14) electroless coating solution. The copper provides an intermediate base layer for subsequent metal deposition through electroless or electroplating process onto the finished part. As a multi-step process, however, the requirement of pre-coating with copper has yet to receive widespread commercial acceptance by magnesium parts manufacturers, other than for use as decorative magnesium parts requiring chrome finishes.

SUMMARY OF THE INVENTION

To overcome at least some of the disadvantages associated with conventional electroless plating processes, the present invention provides a plating process for the deposition of metal coatings directly on magnesium and magnesium alloys using a nickel hydrate plating bath.

Preferably, the plating bath includes a nickel hydrate compound, and more preferably one or more of a nickel acetate tetrahydrate, a nickel sulfate hexahydrate and/or a nickel sulfamate tetrahydrate.

In one preferred mode, nickel phosphorous or nickel phosphorus alloys are deposited as a metal coating layer on magnesium and/or magnesium substrates, either with or without first removing all or part of any magnesium oxides therefrom. Such alloys include, without restriction, nickel phosphorous-zinc alloys and nickel phosphorous tungsten alloys.

In one possible method, a plating bath is prepared as a single component solution which includes a suitable hydroxide in an amount selected to provide the bath with pH of at least 9, preferably 10.5 to 14, and most preferably between about 11 to 14. The solution is heated to a temperature of greater than about 35° C., preferably at least about 50° C. and most preferably between about 65° C. to 80° C. The process plating bath solution preferably is provided with between about 5 to 50 g/L, and preferably 13 to 38 g/L of one or more hydroxides, such as sodium hydroxide and/or ammonium hydroxide, with ammonium hydroxide being more preferred. The hydroxides are provided at a level selected to maintain the solution pH in the alkaline region.

Preferably, the solution also includes 5 to 20 g/L and more preferably 6 to 13 g/L of one or more of nickel hydrate compound such as nickel acetate, nickel acetate tetrahydrate, nickel sulfamate tetrahydrate and/or nickel sulfate hexahydrate.

10 to 30 g/L and preferably 17 to 24 g/L of a combination of compounds of sodium citrate tribasic dihydrate and sodium hypophosphite hydrate are also provided maintaining a sodium citrate tribasic dihydrate level, to maintain the Ni in solution. Optimally, the solution may include upto 15 g/L of a zinc hydrate compound, such as zinc sulfate heptahydrate.

Most preferably, the solution is substantially free of chloride salts in any form. Accordingly, in one aspect, the present invention resides in a process for electroless plating a plating metal on a substrate, and preferably a magnesium or magnesium alloy substrate, where the method comprises preparing a plating bath solution comprising: 6 to 13 g/L of a nickel hydrate compound; 15 to 25 g/L sodium hydrate compound, for stability and plating; 10 to 50 g/L of ammonium hydroxide to maintain an alkaline pH; optionally sodium hydroxide in an amount upto 3 g/L; heating the bath solution to a temperature of at least 50° C.; and immersing the substrate to be plated in the heated solution.

In another aspect, the present invention relates to a method for the electroless plating of nickel or a nickel alloy on a magnesium or magnesium alloy substrate, said method comprising, preparing a plating bath solution comprising: 7 to 15 g/L of a nickel hydrate compound, where the nickel hydrate compound being selected from the group consisting of nickel acetate tetrahydrate, nickel sulfate hexahydrate and nickel sulfamate tetrahydrate; 15 to 25 g/L sodium hydrate compound, the sodium hydrate compound comprising at least one of sodium citrate tribasic dihydrate and sodium hypophosphite hydrate; 5 to 50 g/L of ammonium hydroxide; sodium hydroxide in an amount upto 3 g/L; heating the bath solution to a temperature of at least 50° C.; and immersing the substrate in the heated solution.

Accordingly, in one aspect, the invention provides a plating process for the depositing nickel phosphorous and nickel phosphorous alloy coatings directly on magnesium and magnesium alloys using a plating bath comprising a nickel hydrate compound and one or more of nickel acetate tetrahydrate, a nickel sulfate hexahydrate and/or a nickel sulfamate tetrahydrate. The plating bath is prepared as a single component solution having a pH of 10 or more and heated to a temperature of at least 35° C.

In another aspect, the invention provides a process for electroless plating a plating metal on a substrate comprising, preparing a plating bath solution having a pH of at least 9, the bath solution comprising: 7 to 15 g/L of a nickel hydrate compound; 15 to 25 g/L sodium hydrate compound; 10 to 50 g/L of ammonium hydroxide; and optionally sodium hydroxide in an amount upto 3 g/L; heating the bath solution to a temperature of at least 50° C.; and immersing the substrate to be plated in the heated solution.

In yet a further aspect, the invention provides a method for the electroless plating of nickel or a nickel alloy on a magnesium or magnesium alloy substrate, said method comprising, preparing a plating bath solution having a pH of between 10.5 and 14, the bath solution being substantially free of chloride salts and comprising: 6 to 13 g/L of a nickel hydrate compound, the nickel hydrate compound being selected from one or more of the group consisting of nickel acetate tetrahydrate, nickel sulfate hexahydrate and nickel sulfamate tetrahydrate; 17 to 24 g/L sodium hydrate compound, the sodium hydrate compound comprising at least one of sodium citrate tribasic dihydrate and sodium hypophosphite hydrate; 13 to 38 g/L of ammonium hydroxide and/or sodium hydroxide; heating the bath solution to a temperature of at least 50° C.; and immersing the substrate in the heated solution.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides for a process for the metal coating or plating of magnesium and magnesium alloy substrates or component parts. More preferably, the invention provides a bath solution for the electroless deposition of a nickel phosphorous and/or nickel phosphorous alloy coating layers on a magnesium or magnesium alloy substrate either with or without requiring a first step of magnesium oxide layer removal prior to immersion of the substrate in the plating bath.

The applicant has appreciated that suitable phosphorous alloys to be used in the plating process includes without restriction, nickel phosphorus (Ni—P), nickel phosphorus zinc (Ni—P—Zn) nickel zinc phosphorus (Ni—Zn—P) and nickel tungsten phosphorus (Ni—W—P). In particular, the nickel/nickel alloys may be successfully deposited or magnesium at good rates of deposition using an electroless deposition process with the plating bath having pH of at least 9, and most preferably between from a pH of 11 to pH 14 at room temperature.

The plating bath is preferably substantially free of chloride salts and provided as a single solution component which includes one or more nickel hydrate compounds selected to provide nickel ions in solution, one or more sodium hydrate compounds selected to stabilize the bath and reduce the nickel ions to nickel metal deposits at the magnesium/magnesium alloy part surface, and one or more suitable hydroxides, in an amount selected to provide the bath with the desired alkalinity.

The applicant has appreciated that the bath chemistry of the plating bath allows for reaction not only with unoxidized magnesium, but also with magnesium oxide surfaces, and allow the formation of good nickel and nickel alloys coatings with substantially direct uninterrupted surface contact across the substrate including these surfaces compromised by magnesium oxide.

In a preferred process, the temperature of the plating bath is elevated to a temperature greater than about 50° C. and most preferably to between about 68° to 74° C., and with the pH level decreasing slightly. The applicant has appreciated that the use of an alkaline solution reduces the rate of oxide formation on any oxide-free magnesium surfaces of the substrate. This allows the enhanced deposition of nickel and/or nickel alloys to form an intermetallic bond with clean magnesium surfaces whilst effecting the deposition of the coating over any existing oxidized surfaces. As such, the electroless deposition bath may advantageously be used to form a continuous uninterrupted nickel or nickel alloy coating over the entire surface of the finished magnesium part.

Since the bath solution used in the current process is highly alkaline, the plating bath solution further has a limited corrosive effect on the magnesium surface during the coating process. As a result, the surface of the substrate or part is dimensionally unchanged under the nickel and nickel alloy coating. The nickel and/or nickel alloy coating which is formed provides a continuous uniform, uninterrupted surface over the entire surface of the part. Further, in one preferred process the coating may be formed so as to substantially fully encapsulate the part, thus isolating the magnesium or magnesium alloy from direct contact with other metals in the presence of any potential electrolyte and thereby inhibiting or preventing any potential galvanic corrosion. At the same time the coated magnesium substrate maintains all of its electrical and thermal properties in a multi-metal structure.

In one process of manufacture, it is envisioned that the electroless deposition of a nickel coating may be provided as a final treatment on a finished magnesium component or part, prior to its assembly into a final structure. Since the nickel and nickel alloy electroless coating process is tolerant to oxidized magnesium surfaces, a chemical cleaning bath of tartaric acid or sulfuric acid may optionally be provided as a pre-treatment step for use in removing magnesium oxide from the substrate. In the alternative, or in addition, one or more mechanical processes as for oxide removal from the substrate such as abrasion, grinding or the like, may also be used on exterior and/or easy to reach contact surfaces of the substrate.

Sample Baths

Sample bath solutions S1 to S7 were prepared in accordance with the following Table 2 for the electroless nickel phosphorous deposition on a test magnesium alloy substrate composition AZ91D. In the sample plating process, the electroless plating solution was based on a single solution that is stable until it is heated to deposition temperature. The applicant has appreciated that deposition rates with nickel phosphate (Ni—P) coatings have proven commercially acceptable where deposition occurs comparatively fast, with good surface deposition completed in under 1 minute. As alloy substrates or parts are added, the coating deposition rate will tend to decrease, requiring minutes to complete and with most commercially acceptable coatings achieved in under 5 minutes. In the test samples, bath solution S7 zinc deposition was however shown to achieve acceptable coatings at a slower rate closer to 10 minutes.

In the test solution a magnesium alloy substrate was selected as per the following Table 1:

TABLE 1 The composition of AZ91D magnesium alloys (in wt. %) Alloy Al Zn Mn Ni Cu Si Fe Magnesium AZ91D 8.3-9.7 0.35-1.0 0.15-0.5 <0.002 <0.03 <0.10 <0.005 Balance Sample deposition baths for forming nickel/nickel plating test magnesium substrates were prepared according to test bath formulations S1 to S7 shown in Table 2:

TABLE 2 Electroless nickel phosphorous [Ni—P] thin film deposition bath formulations. Deposition temperature tested: 68-74° C. Chemical Chemical Bath Composition (g/L) Name Formula Bath S1 Bath S2 Bath S3 Bath S4 Bath S5 Bath S6 Bath S7 Nickel acetate Ni(C₂H₃O₂)₂· 9.940 9.940 tetrahydrate 4H₂O Nickel sulfate NiSO₄·6H₂O 10.499 10.499 6.299 hexahydrate Nickel Ni(H₂NSO₃)₂· 12.899 12.899 sulfamate 4H₂O tetrahydrate Zinc sulfate ZnSO₄·7H₂O 4.594 heptahydrate Sodium citrate Na₃C₆H₅O₇· 23.500 23.500 23.500 23.500 23.500 23.500 23.500 tribasic 2H₂O dihydrate Sodium NaPH₂O₂· 17.500 17.500 17.500 17.500 17.500 17.500 17.500 hypophosphite H₂O hydrate Sodium NaOH 1.250 1.250 1.250 hydroxide Ammonium NH₄OH 12.5 12.5 12.5 37.5 37.5 37.5 40.0 hydroxide Average pH before use (20° C.) 11.81 11.98 11.85 11.86 11.92 11.93 11.69

From test samples, it was observed that the inclusion of ammonium hydroxide advantageously facilitated deposition on the magnesium substrate. The addition of ammonium hydroxide further advantageously resulted in an increase in the alkaline level of the bath, and acted to offset any acidic components of the bath solution. More preferably the bath solution is kept substantially free of chloride (Cl⁻) ions which could result in rapid decomposition of the bath. In particular, free chloride ions are frequently derived from Nickel Chloride (Ni—Cl₂), and which are generally believed unsuitable for use in solutions for the electroless deposition of coatings on magnesium.

In the test samples, Bath S7 was the only plating bath containing zinc. The applicant has appreciated that as the bath pH is increased, so does the amount of zinc that can be carried within the bath (see Table 3 below). Preferably, however, the zinc sulfate heptahydrate is maintained at a level below 60% (cut) of the Nickel present in the plating bath. In the resulting test sample, the coated surface was shown to have a consistent 20% +/−zinc content.

TABLE 3 Constituent atomic percentage concentration of the elements in the films as a function of metalizing bath pH value (bath temperature kept constant at 85° C.). pH Nickel Zinc Phosphorous 6.3 70 4 26 7.4 70 10 20 9.5 71 19 10

As an optional process step, it is envisioned that prior to immersion in the plating bath solution, the magnesium/magnesium alloy substrate or part to be coated is pre-treated in an acid cleaning bath to first remove magnesium oxide. Suitable cleaning bath solutions for such magnesium and/or magnesium alloys include tartaric and/or sulfuric acid baths as follows:

Tartaric Acid Bath

Chemical Formula Concentration Tartaric Acid C₄H₆O₆ 30 to 60 g/L (approx. 53.0 g/L preferred)

Sulfuric Acid Bath

Chemical Formula Concentration Sulfuric Acid H₂SO₄ 15 to 30 mL/L (approx. 20 mL/L preferred)

Commercial Process

The applicant has appreciated that in one preferred process of manufacture, magnesium and magnesium alloy parts or component blanks may undergo the electroless plating of nickel and nickel alloys by immersion, whereby:

-   -   1. An alkaline plating bath is provided with a pH level of 11 to         14 at elevated temperatures.     -   2. The coating process conducted in a plating bath at         temperature between 60° to 85° C.     -   3. Preferred plating bath formulations are as per Table 2, and         most preferably Bath formulations S1, S4, S5, S6 and S7.     -   4. The plating bath is provided as a stable single formulation,         as contrasted with a two component system.     -   5. The inclusion of ammonium sulphate in the plating bath may         result in rapid bath deteriorating. As such ammonia used in the         bath is most preferably derived through the use of ammonium         hydroxide.     -   6. A tartaric acid or sulfuric acid bath formulation may         optionally be used as part of a pre-plating de-oxidation process         to remove the magnesium oxide from the magnesium part.     -   7. The plating solution is most preferably chloride free, with         nickel chloride salts being avoided in the process.

In preferred commercial process for the production of nickel coated magnesium parts and components, finished magnesium blanks ready for assembly are first cleaned of foreign materials.

The cleaned blanks are next dipped into a tartaric or sulfuric acid cleaning bath described, above, and which is provided at room temperature for less than about 45 seconds to remove any formed magnesium oxide. Preferably, whilst so immersed, the blank is manipulated and/or agitated to better effect the acid bath contact with all part surface areas.

Following oxide removal, blank is thereafter transferred to the electroless plating bath made up of one of formulations S1-S7, and which has been heated to a temperature between 50° C. and 85° C. and preferably at about at 68° to 74° C. The blank is placed in the plating bath such that all surface areas to be nickel coated are provided in contact with the solution. Optionally the part may be manipulated and/or agitated in the plating solution, and/or a volume of the plating solution may be pumped on, into or through the blank. Depending on the solution formulation and the desired plating thickness, the blank is left in the plating solution for a period of time of from between about 1 to 10 minutes, until the desired surface nickel coating build-up is achieved.

Following plating, the part is then removed from the electroless solution, rinsed in de-ionized water at room temperature; and dried, after which it is ready for use/installation.

While the detailed description describes and illustrates various preferred embodiments, the invention is not limited strictly to the precise embodiments which are disclosed. Modifications and variations will now occur to persons skilled in the art. For a definition of the invention, reference may be had to the appended claims. 

We claim:
 1. A process for electroless plating a plating metal on a magnesium or magnesium alloy substrate comprising, preparing a plating bath solution having a pH of at least 9, the bath solution comprising: 7 to 15 g/L of a nickel hydrate compound; 15 to 25 g/L sodium hydrate compound; 10 to 50 g/L of ammonium hydroxide; and optionally sodium hydroxide in an amount upto 3 g/L; heating the bath solution to a temperature of at least 50° C.; and immersing the substrate to be plated in the heated solution.
 2. The process as claimed in claim 1, wherein the nickel hydrate compound is selected from the group consisting of nickel acetate tetrahydrate, nickel sulfate hexahydrate and nickel sulfamate tetrahydrate.
 3. The process as claimed in claim 1, wherein the sodium hydrate compound comprises at least one selected from the group consisting of sodium citrate tribasic dehydrate and sodium hypophosphite hydrate.
 4. The process as claimed in claim 1, wherein the bath solution comprises ammonium hydroxide in an amount of between about 10 to about 50 g/L.
 5. The process as claimed in claim 4, wherein the bath solution comprises sodium hydroxide in an amount of 0.1 to 1.5 g/L.
 6. The process as claimed in claim 2, wherein the bath solution has a pH of between 11 and 14, and comprises between about 12 to 40 g/L ammonium hydroxide.
 7. The process as claimed in claim 6, wherein the substrate comprises a magnesium alloy substrate.
 8. The process as claimed in claim 1, wherein the solution is heated to a temperature of from about 60° C. to about 85° C.
 9. The process as claimed in claim 1, wherein the solution further includes 3 to 7 g/L zinc sulfate heptahydrate, and which preferably maintained at a level below 60% (cut) of the Ni present in the bath.
 10. The process as claimed in claim 9, wherein the nickel hydrate compound comprises 4 to 7 g/L nickel sulfate hexahydrate.
 11. A method for the electroless plating of nickel or a nickel alloy on a magnesium or magnesium alloy substrate, said method comprising: preparing a plating bath solution having a pH of between 10.5 and 14, the bath solution being substantially free of chloride salts and comprising: 6 to 13 g/L of a nickel hydrate compound, the nickel hydrate compound being selected from one or more of the group consisting of nickel acetate tetrahydrate, nickel sulfate hexahydrate and nickel sulfamate tetrahydrate; 17 to 24 g/L sodium hydrate compound, the sodium hydrate compound comprising at least one of sodium citrate tribasic dihydrate and sodium hypophosphite hydrate; 13 to 38 g/L of ammonium hydroxide and/or sodium hydroxide; heating the bath solution to a temperature of at least 50° C.; and immersing the substrate in the heated solution.
 12. The method as claimed in claim 11, wherein the bath solution comprises ammonium hydroxide in an amount of between about 10 to about 40 g/L.
 13. The method as claimed in claim 11, wherein the bath solution comprises sodium hydroxide in an amount of upto 3 g/L and preferably 0.1 to 1.5 g/L.
 14. The method as claimed in claim 13, wherein the bath solution comprises between about 12 to 40 g/L ammonium hydroxide.
 15. The method as claimed in claim 11, wherein the substrate comprises a AZ91D magnesium alloy.
 16. The method as claimed in claim 11, wherein the solution is heated to a temperature of from about 68° C. to about 74° C.
 17. The method as claimed in claim 11, wherein the solution further includes 3 to 7 g/L zinc sulfate heptahydrate which is maintained at a level below 60% cut of the nickel in the bath.
 18. The method as claimed in claim 17, wherein the nickel hydrate compound comprises 4 to 7 g/L nickel sulfate hexahydrate.
 19. The method as claimed in claim 11, wherein said sodium hydrate compound comprises sodium citrate tribasic dihydrate in an amount selected to maintain nickel in solution, and further wherein said plating bath comprises sodium hypophosphite in an amount selected to reduce nickel ions to metal at the surface of the substrate. 